Geothermal Heat Exchanger Research Articles

Thermal interaction of slender geothermal boreholes with creeping groundwater flows

In the presence of groundwater flows, more compact and affordable geothermal heat exchangers can be constructed thanks to the increased heat transfer capabilities of the ground. To correctly design them, theoretical models for the thermal interaction of geothermal boreholes with aquifers are required. By using matched asymptotic expansion techniques, a new theoretical model for the case of creeping groundwater flows is developed in the present work. This new model improves by an order of magnitude the accuracy of the state of the art, leading to absolute (relative) errors below 0.03 (2.3%). Additionally, the mathematically-rigorous and physically-sound derivation of the proposed model allows, for the first time, the assessment of the theoretical and conceptual merits and limits of the state of the art.

Aplicación de métodos eléctricos y electromagnéticos, combinados con el análisis de propiedades térmicas, a la caracterización y del recurso geotérmico de baja y media entalpía

The relationship between the thermal properties of the ground and the efficiency of geothermal installations located in the ground is well known. The different geologies and subsurface structures have a decisive influence on the behavior of a given location with respect to heat exchanges. The electrical and electromagnetic methods of geophysical prospecting can be useful in determining the compositions and structures present in a given area in order to be able to perform energy modeling that will help to design more precisely the ideal geothermal heat exchanger for that area. In this work we study the relationship, observed in different studies, between the geological structure and the electromagnetic magnitude measurements collected from these prospecting methods. Possibilities are also explored in contact with the idea of relating electromagnetic and thermal magnitudes in the search for new ideas to develop more accurate methods of geophysical prospecting of the geothermal resource.

Heat extraction analyses and energy consumption characteristics of novel designs of geothermal borehole heat exchangers with elliptic and oval double U-tube structures

The borehole heat exchanger (BHE) is an efficient and economic tool for extracting geothermal energy within ground source heat pumps (GSHPs). The layout and cross-section tube configuration of the BHE is one of the crucial bottlenecks that strongly influenced the effectiveness of the BHEs, which recently attracted numerous attention.Hence, the interest in retrofitting cost-efficient configurations of BHEs is recently regarded as an improvement pathfor maximizing geothermal energy production. In this study, novel geothermal borehole heat exchangers with elliptic and oval double U-tube (2U) configurations are investigated and compared to a traditional circular 2U BHE.This paper explains the development of traditional circular 2U-tube BHEs that makes an optimally designed model for maximizing the heat extraction rates and minimizing the energy consumption characteristics and BHE number. A 3-D numerical model was developed and validated by relevant field experiments (Tianjin geothermal project). The numerical study was performed on four cases corresponding to different widths of elliptic and oval 2U BHEs configurations. Furthermore, a sensitivity analysis was performed to parametrically investigate the impacts of key design and operational parameters on heat extraction performance. Additionally, a 3-D numerical model coupled with a finite line heat source model was also developed to perform case studies for energy consumption analysis for two goals of optimization. The results show that the oval 2U BHE (type II) outperformed the other investigated 2U BHE configurations, which increased the maximal and mean heat transfer rate by up to 54.0% and 12.30%, respectively, compared to circular 2U BHE. While the elliptic 2U-tube BHE (type II) was minimal pressure drop and energy consumption rates, which showed a pressure drop of 36.20% lower than the circular 2U BHE. In the case study findings, the utilization of elliptic 2U-tube BHE (type II) can not only reduce the BHE number by 6.70% but also minimize the energy consumption of the water pump and the entire system by 36.9% and 8.2%, compared to that of traditional circular 2U-tube BHE, respectively.

Modeling and performance analysis of geothermal energy based air conditioning in building in Iraq

In many desert places, like Iraq, summer lasts a long time, with a mean daily high temperature of 45 °C. Buildings use a lot of electricity to run the air conditioning systems, which is a big part of the total amount of electricity produced. Everywhere in the hot and dry subtropics, a passive method of cooling is a real way to save money on air conditioning. One of these methods, called earth pipe cooling uses the relatively constant temperature of the ground below to cool indoor spaces without using any mechanical equipment. This reduces or eliminates the building's overall energy use and impact on the environment. In a space that was 3 × 3 × 5.5 m, the temperature of the ground and the efficiency of a 31-m-long, 3-m-deep, 6-inch-wide ground heat exchanger were measured. The effect of weather on the temperature of the air coming out of a building depends on the strength of the sun, the temperature of the ocean, and the relative humidity, all of which change depending on where the building is located. Also, the output air temperature and thermal performance change concerning air moving from 4 to 6 m/s. Using MATLAB and ANSYS FLUENT, the researchers carried out the simulations of the cooling system and it was found that the theoretical results were within 1.3% of the actual results. The results showed that climate variables have a big effect on the ground temperature down to a depth of 0.25 m, but at a depth of 3 m, the ground temperature remains the same (21 °C) all year long. Also, the results exhibited that the temperature of the air coming out of the system increased from 23 °C to 32 °C as the air moved faster through the system. Since the geothermal heat exchanger is most effective at cooling at a speed of 4 m/s, slowing the flow speed makes it work better. By using a geothermal heat exchanger, the difference in temperature between the inside and outside air could be cut by 10–16 °C. The data also showed that the speed and diameter of the air leaving the pipe made it hotter, while length, depth, and conductivity made it cooler.

Solid state thermoelectric power generator with passive heating and cooling

The use of solar energy to electrical power generation becomes an opportunity for socioeconomic improvement for regions affected by excessive solar radiation, as well as the Brazilian Northeast. This work proposes a new way of harnessing solar energy to generate electricity with a higher percentage of nationalization. A prototipe of a solid state thermoelectric generator is presented. The project considers two different natural sources to produce temperature difference. They are solar concentration for heating and geothermal heat exchanger for cooling. Boundary conditions based on the bibliographic review was produced artifitially. The system performance was analyzed in real time through a computerized acquisition system. The thermal behaviour was registered in a thermographic picture. The project resulted in the electric power generation of approximately 3W delivered to a resistive load of 4 Ω, under a temperature difference of approximately 60 °C. Comparated to a regular photovoltaic cells that produces 3,8W at most, this propose can become an alternative to commertial systems. The novelty of this work is the combination of the thermoelectricity, solar heating and geothermal cooling technologies, necessariously.

Design and optimization of heat extraction section in energy tunnel using simulated annealing algorithm

Energy tunnel has attracted increasing interest in preventing tunnel frost damage and providing energy for space conditioning through utilizing geothermal energy from ground source heat pump (GSHP) system. The energy lining system, composed of heat pump units, lining buried with geothermal heat exchangers (GHEs), and supply pipes, is one of the most popular energy tunnel systems. Despite their good potential for geothermal energy exploitation, studies on their design approach are still insufficient, particularly for cold region energy tunnels. In this work, we propose an innovative design framework for optimization of the heat extraction section in the energy tunnel by combing numerical simulation and Simulated Annealing (SA). The numerical model reproduces a horseshoe-shaped tunnel segment equipped with an energy lining system, generating heat power activated by circulating heat carrier fluid. And the simulation provides key input parameters, involving heat extraction power, the ground temperature field, and heating demand, for the algorithm, hence the heat extraction section range is optimized using SA. Influences of four design parameters (i.e., ground temperature, inlet fluid temperature, circulating fluid flowrate, and pipe spacing) and three axial temperature distribution characteristics (i.e., overall temperature along the tunnel axis, airflow characteristics, and temperature difference between the tunnel entrance and exit) dominating temperature profiles along the tunnel axis on the optimal range design are investigated. Major findings include: (1) the optimal range is sensitive to the temperature field along the tunnel axis, and the normalized optimal length decreases by 91.3% with the normalized feasible range increases 1099.6% as the average temperature inside the tunnel rises from −2.5 °C to 5 °C; (2) increasing flowrate and lowering the temperature of the inlet fluid can yield a reduction in the optimal length of up to 29.2% and 64.3% separately; (3) a relatively sparse pipe arrangement can save cost without comprising the optimal length. This study presents a smart and efficient approach for designing the heat extraction section of the energy tunnel, which is promising in more complex conditions and capable of guiding practical applications.

A preliminary Techno-economic feasibility study of geothermal heat exchange system for buildings in India

A preliminary Techno-economic feasibility study of geothermal heat exchange system for buildings in India

Optimizing and simulating cooling of electric transformer room utilizing genetic algorithm to reduce electricity/water demand by incorporating borehole ground heat exchangers

BackgroundIn this study, the optimization of the transformer room cooling systems was discussed and investigated. According to the cooling requirements of the transformer room (airflow and maximum temperature), a water-cooled chiller and air conditioner were used. MethodsBy using numerical analysis, the mass/energy of the cooling tower was studied. To improve the effectiveness of the cooling strategy, the cooling tower was removed and the ground heat exchanger was incorporated into the cooling system. ResultsBased on the numerical analysis, if the ground heat exchanger is added, the electricity and water demand was reduced by 12.65 kWh and 1.94 m3 per square meter. The performance of the geothermal heat exchanger depends on the number of pipes, but the amount of dependence decreases with the increase in the number of pipes. To find the best number of tubes, the genetic algorithm was used and it was observed that if the number of tubes reaches sixteen, for every one percent increase in costs, the heat transfer in the geothermal exchanger increases by 2.54 percent.

Experimental study on convective heat transfer of an open-loop borehole heat exchanger

Open-loop borehole heat exchanger (OBHE) is a single well geothermal heat exchanger with an open-loop structure that can realize the geothermal energy extraction without mining the geothermal water. In this paper, a sandbox experiment is designed to simulate the convective heat transfer process in the reservoir area of OBHE. The mechanism of convective heat transfer in the reservoir area is studied, and the key factors that affect the convection heat transfer intensity are analyzed. The results show that the convection heat transfer of OBHE in the reservoir area is affected by both the driving effect of fluid flow inside the screen tube and the buoyancy effect. In the forward flow mode, the two effects have the opposite direction. While in the backward mode, the two effects have the same direction. The backward flow mode is more conducive to convective heat transfer. In addition, many factors influencing significantly the convective heat transfer of OBHE include inlet temperature, inlet flow rate, reservoir temperature, fluid flow direction and inner tube diameter.

A geothermal energy heat exchange system suitable for abandoned mines and its optimization

A geothermal energy heat exchange system suitable for abandoned mines and its optimization

The Characteristics, Methods, Trends and Applications of Intelligent Systems

Interaction between intelligent systems and their human operators in dynamic, shifting, and unpredictable natural and social settings is novel area of study within the topic of intelligent computing systems. Robots of the past were not effective at making decisions on their own; instead, they routinely carried out the same set of actions in the same situations because they believed that the world was predictable. Nowadays, decisions may be made quickly and effectively by intelligent systems in practical settings. Modern intelligent systems include characteristics such as intelligent search and optimization, artificial evolution, and autonomous decision support that are unavailable in a traditional information system. An in-depth analysis of the methods used to create intelligent systems is presented in this paper. These techniques are often categorized as either artificial intelligence or soft computing. Some examples include the use of a neural network, fuzzy logic, a hybrid system, or a swarm of intelligent insects. In addition, this article gives an overview of two applications of intelligent systems and technologies such as Geothermal Heat Pumps (GHPs) and Heat exchangers (HEXs).

Improving the Efficiency of a Conventional Vapor-Compression Refrigeration System used for Geothermal and Evaporative Cooling Techniques: Case Study in Iraq

A conventional vapor compression refrigeration cycle is among the most effective technology in refrigeration systems. In the present experimental study, thermal performance and the effect of the condenser temperature have been analyzed. With the decrease in the condenser temperature, the overall performance of the refrigeration cycle is increased. However, the cooling power and efficiency of conventional vapor pressure air conditioning units can experience a significant reduction when operating in extreme weather (hot and dry). This drop is mainly affected by the increase in the temperature (and pressure) of the condenser with an increase in the ambient air temperature. Unfortunately, Iraq experiences the most extreme summer seasons especially in the months of June and July when the temperature reaches or exceeds 50° C. So, ground cooling has been used in areas with a hot environment for split system air conditioners. Evaporative cooling was also performed to lower the coolant temperature of the AC unit used inside the condenser area. Experimental results showed that when using a geothermal heat exchanger, the temperature of the condenser is reduced from 116 to 110 ° C and the coefficient of performance (COP) is improved by 41%. In addition to this when the system uses evaporative cooling the temperature of the condenser is reduced from 110 ° C to 88° C. Moreover, a 65% improvement was made in the COP of the conventional vapor compression refrigeration cycle. Furthermore, with a decrease in the evaporator temperature from 6 to 3.5 °C there was an increase in refrigeration capacity by an average of 52%.

ОБҐРУНТУВАННЯ ПАРАМЕТРІВ СПОСОБУ ВИКОРИСТАННЯ ГЕОТЕРМАЛЬНОЇ ЕНЕРГІЇ ДЛЯ ЦІЛОРІЧНОГО ЗАБЕЗПЕЧЕННЯ ВОДОЮ БЕТОНОВИРОБНИХ ПІДПРИЄМСТВ

Purpose: Substantiating the parameters of the method of using alternative energy sources for the year-round supply of thermostabilized water to enterprises producing concrete, due to establishing the regularities of the functioning of the geothermal heat exchanger. Methodology: a comprehensive approach, which includes theoretical studies of heat transfer from the massif to the geothermal heat exchanger, calculation and analytical method for determining the main parameters of the geothermal heat exchanger. Results: a subsurface thermal energy extraction installation was developed, due to the use of a geothermal heat exchanger, which will allow year-round production of thermostabilized water from shallow non-potable reservoirs. Scientific novelty: the study of heat transfer from the massif to the geothermal heat exchanger made it possible to determine the dependence of the length of the geothermal heat exchanger on its diameter. Water heating to 8°С in winter and water cooling to 20°С in summer will be provided by a geothermal heat exchanger with a length of 28 m and a pipe diameter of 0.5 m. Further increase in the diameter of the pipes will lead to a decrease in the efficiency of the TRP. Practical significance: obtaining thermostabilized water throughout the year will create conditions for year-round operation of concrete-making enterprises for the intensive restoration of Donbass.

Ground Source De-Icing and Snow Melting Systems for Infrastructure

Thermal de-icing and snow melting methods to control winter conditions on surfaces of transport infrastructure offer several advantages compared to conventional techniques. These include the automated control of safe surface conditions, avoidance of chemicals and their environmental impact and prolongation of the life span of the infrastructure. Hydronic heat transfer systems can take advantage of collection of solar energy mainly during summertime and seasonal storage of thermal energy by geothermal heat exchange. Making use of these renewable resources in combination with energy storage enables savings in primary energy. In June 2021, the International Energy Agency (IEA), initiated a project related to utilization of ground thermal energy sources for de-icing of surfaces in transport infrastructures. The present paper gives a first overview of the project goals and methods.

Thermal Analysis of U-Type Geothermal Heat Exchangers of Complex Configurations Using Novel Reduced-Order Models

Thermal Analysis of U-Type Geothermal Heat Exchangers of Complex Configurations Using Novel Reduced-Order Models

Numerical study on heat transfer and flow characteristics of coaxial geothermal heat exchangers with helical finned inner tubes

In recent years, geothermal energy as well has become the first choice for building heating. In order to improve the heat transfer performance of the coaxial GHE, the flow characteristics and heat transfer characteristics of the coaxial GHE when the inner tube of the coaxial GHE is assembled with HF of different width ratio (a), thickness ratio (b) and pitch ratio (S) are numerically studied in this paper. Water is chosen as the working medium, and the Reynolds number range is 10000–60000. HF have a significant impact on heat exchanger performance by changing the fluid flow structure and improving the efficacy of heat transfer between the fluid and the wall. Numerical results show that as the Reynolds number increases, the Nusselt number (Nu) of coaxial GHE with different a, b, and S increases, while the friction factor(f) and thermal enhancement factor (TEF) decreases. The Nu and f of the coaxial GHE with HF of different a, b, and S on the inner tube are higher than those of the coaxial GHE with smooth inner tube. The change in S of HF has a greater effect on Nu and f than the changes in a and b of HF have on Nu and f. To discover the enhanced heat transfer mechanism of the coaxial GHE with HF assembled on the inner tube, flow lines, velocity, turbulence, turbulence, and temperature distribution are provided. According to the analysis, the blocking of the HF causes the fluid to appear as a spiral rotating flow structure, which improves fluid mixing and heat exchange between the fluid and the wall. Increases in both a and b of HF can improve the heat transfer strength of the fluid to the wall, while decreases in S of HF can hasten fluid mixing. Finally, two correlations are established based on the numerical simulation data to predict Nu and f as functions of design parameters in turbulent flow states.

Experimental Analysis for the Performance of a Geothermal Heat Exchanger Made of Environmentally Friendly Materials

Geothermal heat exchanger (GHE) technology is commonly used in residential and small non-residential buildings, and examples of innovative compact GHE designs are provided. The essential physical properties and processes involved in shallow GHE behavior are described, some of which may be more complex than those required for other types of GHE. This paper discusses the use of a geothermal heat exchanger to extract underground energy for heating and cooling systems and focuses specifically on designing a ground heat exchanger made of environmentally friendly materials suitable for working in the zone of Babylon Governorate in Iraq. At different burial depths, three systems of horizontal heat exchangers were designed. The heat exchanger was designed with pipes made of peroxide cross-linked polyethylene (PEX), in which the pipes were buried to a depth of 2 m under the soil. Also, another system was designed where the pipes are made of multi-layer composite pipe (Mlcp) at a burial depth of 2.5 m and another layer at a depth of 3 m. In this research, several tests were carried out to reach the conditions in which the ground heat exchanger is at its best performance. The effect of buried depth on heat removed from the space has been studied. This test was studying at a flow rate equal to (4 lpm) and the inlet temperature of water equal to approximately (50℃). The effect of changing the flow rates on heat removed from the ground heat transfer system is being studied. Buried depth of PEX pipes is 2m, inlet water temperature (50℃) is chosen as a case study to analyze this effect. The first experimental has shown the heat removed is equal to 2200.942 watts at 8.30 am when buried at a depth of 2m, while it reaches 3679.474 watts at a depth of 3 m. The results show the maximum coefficient of performance (COP) at (3 lpm) is 12.897 and it becomes 5.99 at 4 LPM. Also, the results show that the COP of PEX pipes can be higher than that of its counterparts when pipes are made from MLCp.

One-Year Monitoring of a Ground Heat Exchanger Using the In Situ Thermal Response Test: An Experimental Approach on Climatic Effects

The use of renewable energies, and of geothermal energy in particular, is increasingly being applied in Germany and Europe for the development of new residential districts. The use of geothermal borehole heat exchangers (BHE), in combination with ground-source heat pumps (GSHP), represents an important part of shallow geothermal systems, which are used, among other systems, in urban areas due to their small space requirements. Over the course of planning BHE systems, performance must be determined via the parameters of thermal conductivity, thermal capacity, undisturbed ground temperature, and borehole thermal resistance. These can be identified by the experimental approach known as thermal response testing (TRT). The thermal parameters change due to the influences of the seasonal temperature fluctuations that take place in the ground. In this paper, a pilot double-U BHE heat exchanger field with a depth of 120 m was investigated from this perspective. TRT was carried out using monthly measurements taken over the period of one year using an electrically powered mobile TRT device. The evaluation of the individual tests was carried out using the line-source, moving-line-source, and cylinder-source theories. Our results show that the season in which TRT was implemented had an influence on the determined thermal parameters, with better thermal conditions being obtained in winter months. This is especially visible for thermal conductivity, with monthly deviations of 0.1 W/(m∙K), independent of the evaluation approaches used.

Thermo-hydraulic analysis in geothermal energy walls

A ground source heat pump (GSHP) system provides efficient space heating and cooling and thus is regarded as a contributor to achieve net-zero emissions targets. This study focuses on an economical type of GSHP system – energy walls where earth retaining walls are equipped with pipes to act as geothermal heat exchangers in addition to being geotechnical structures. A detailed numerical investigation is performed to study the long-term heat exchange mechanism between the walls and the surrounding ground. The work highlights the significance of the existence and magnitude of groundwater flow on the temperature distribution of the ground and the thermal performance of energy walls. Compared to the case when there is no subsurface flow, the energy retaining wall can offer up to a 6 % better coefficient of performance (COP) or provide up to 1.3 times higher thermal yield for relatively slow groundwater flow velocities of 0.013 m/d. The COP can improve up to 93 % and thermal yield up to 23 times for high velocities of 2 m/d, even in extremely cooling-dominant thermal demand cases. The study also shows that the ground thermal conductivity plays a crucial role on performance when groundwater flow is minimal, however, as the convective heat transfer resulting from the groundwater flow becomes more dominant, the influence of thermal conductivity gradually diminishes. It is also found that the absorber pipe flow rate has a rather small effect on the system thermal performance, of less than a 10 % COP difference in all studied cases. This suggests that obtaining reliable hydrogeological information specifically on the groundwater flow velocity and direction are crucial for accurate predictions and optimal design for energy retaining walls and large scale GSHP installations. The insights from this study can improve the design and enhance the uptake of retaining walls for efficient heating and cooling of above/underground spaces - critical components in achieving a clean energy future.

An analytical heat transfer model for the mid-deep U-shaped borehole heat exchanger considering groundwater seepage

Ground source heat pump systems consisting of deep boreholes and heat pumps can be used for heating buildings. It is known as an effective way to utilize geothermal resources, however, there is a problem with high development costs. Therefore, it is important to establish a precise heat transfer model to develop a high-efficiency geothermal heat exchanger to reduce the development cost of mid-deep geothermal energy. Most of the existing heat transfer models for deeply buried pipes only consider thermal conductivity and treat the geotechnical soil as a homogeneous medium, or do not consider the stratification and groundwater seepage phenomena, which make the models inaccurate. This study established an analytical heat transfer model considering geotechnical stratification and seepage developed for the U-shaped deep borehole heat exchanger (U-DBHE), and the analytical solution for temperature response is obtained. Using the developed analytical model, we plotted the trends that reveal the relationship between the thermal performance of U-DBHE and various parameters and explored the influence of seepage phenomena on heat extraction performance. The model can also serve as a reference for the development of heat transfer models for buried pipe heat exchangers with groundwater seepage. Given that the proposed analytical model is time-consuming and stable, and can be an efficient tool for U-DBHE design and optimization.